The present invention relates to a method for surface-release coating and a coating agent therefor which is a radiation-curable organopolysiloxane or a composition comprising the same. More particularly, the invention relates to a method for surface-release coating and a coating agent therefor which is a radiation-curable organopolysiloxane or a composition comprising the same capable of being readily cured by the irradiation with radiation such as electron beams and ultraviolet light to form a cured film having surface-releasability so as to be useful as an anti-sticking treating agent of the back surface of pressure-sensitive adhesive tapes and a coating agent of release paper sheets for temporary protection of pressure-sensitive adhesive labels.
Among various types of radiation-curable organopolysiloxane compositions, those most widely used in the above mentioned applications are formulated with an organopolysiloxane having (meth)acryloxy groups in the molecule as the principal ingredient. These (meth)acryloxy-containing organopolysiloxanes are prepared, for example, by the method disclosed in Japanese Patent Kokai 63-135426 and 2-45533, in which the (meth)acryloxy groups are introduced into the molecules of the organopolysiloxane by utilizing the ring-opening addition reaction of epoxy groups or by the method disclosed in Japanese Patent Kokai 48-48000 and 2-163166, in which the hydrosilation reaction is utilized for introducing the (meth)acryloxy groups into the organopolysiloxane molecules.
The (meth)acryloxy-containing organopolysiloxanes prepared by the above mentioned prior art methods can of course be used as such or as the principal ingredient of a radiation-curable organopolysiloxane composition used for surface-release coating. These radiation-curable organopolysiloxanes, however, have a problem due to the strong polarity of the (meth)acryloxy groups. While the radiation-curability of the organopolysiloxane or a composition thereof can be increased by increasing the content of the (meth)acryloxy groups in the organopolysiloxane, namely, the surface tension and the resistance against peeling of the coating layer are increased so much by increasing the content of the (meth)acryloxy groups in the organopolysiloxane to decrease the usefulness of the organopolysiloxane as a coating agent for the surface-release treatment. Moreover, the peeling resistance of the surface-release coating layer formed from the radiation-curable organopolysiloxane or a composition thereof is increased during the lapse of time even though the initial peeling resistance may be sufficiently small.